HF/6-311G^** and B3LYP/6-311G^** calculations of tetrachlorodibenzo-p-dioxins (TCDDs) predicted that the lowest-energy isomer is not the most toxic 2, 3, 7, 8-TCDD isomer but l, 3, 6, 8-TCDD. This is contrary to the AM1 and PM3 predictions which are used for the elucidation of the isomer distribution of the TCDD homologue in combustion-derived samples.